Cytochemical methods have been used to determine the position of substances in tissues and cells. (1) Beta-Glucuronidase has been applied as a cell marker in tetraparental mice. This approach has been used to study cell movement and enzyme exchange between cells. (2) Two new highly fluorescent tissue-reactive dyes (N-substituted naphthalimides) have been used to mark individual neurons in vivo through recording microelectrodes.